The Collaborative International Dictionary of English v.0.48:
cabbage worm \cab"bage worm`\, cabbageworm \cab"bage*worm`\n.
the larva of several species of moths and butterflies, which
attacks cabbages. The most common is the toxic green larva of
a white butterfly, the cabbage butterfly, (Pieris
rap[ae]). The cabbage cutworms, which eat off the stalks of
young plants during the night, are the larv[ae] of several
species of moths, of the genus Agrotis. See Cutworm.
larva of a cabbage butterfly.